I. Field of the Invention
This invention relates to an improved miter clamp. More particularly, the invention relates to a miter clamp which exerts compressive joining force perpendicular to the joint line between two pieces forming a miter joint in a decorative picture frame or the like by engaging the two pieces from the unfinished or back side only, thereby eliminating the possibility of any unwanted marks on either the front or the edge of the frame members.
II. Description of the Prior Art
In fabricating picture frames and the like in which the ends of adjacent frame members are mitered to form right-angle corners, it is generally desirable to minimize or entirely avoid making marks in the front or outside edges of the frame members-- i.e., the portions of the frame that are in view when the frame is hung on a wall. Thus the miter joints in many such assemblies are fastened with glue rather than with nails, which must be driven in from the outside edges of the joint members and are thereafter visible when the frame is hung on a wall.
As is well known, miter clamps are used to hold the mating surfaces of a glued miter joint tightly together while the glue therein sets or cures. For best results, the compressive joining force exerted by such clamps should act in a direction perpendicular to the mating surfaces which form the joint line; if the force is not so oriented, the joint members must be firmly held in position to prevent their slipping with respect to each other during the time the glue is setting.
Miter clamps of the prior art are of three general types. One type consists of an arrangement of adjustable supporting members which surround and firmly hold the entire frame being fabricated; an example of such a clamp is disclosed in U.S. Pat. No. 1,359,597. Such clamps are effective for holding a frame while the glue in miter joints is setting, and they generally do not mark the surface of the frame, but they are bulky and generally of complex construction, and are not practical where only a single joint is to be glued.
In a second type of clamp two frame members forming a miter joint are held in place in two channels, each of which has a fixed side wall and a movable side wall; the position of the latter is adjustable both to handle frame members of various widths and to firmly secure the member being glued in position against its mating member. The principal disadvantage with this type of clamp is that it does not exert joining force in a direction perpendicular to the joint line; rather, the forces exerted are parallel to the long axes of the members being glued, such that optimum conditions for a strong joint are not obtained.
The third type of prior art miter clamp is exemplified by U.S. Pat. Nos. 842,555 and 949,096. In this type of clamp, adjustable pivoted arms engage the outer edges of the two pieces forming a miter joint; joining force is exerted by turning a screw which tends to force the arms together. The joint members are prevented from slipping out of the arms by means of pins or serrated surfaces which dig into the edge of such members. Such pins or serrated surfaces give rise to the principal disadvantage of this third type of clamp, namely, they mark or scar the outer edge surfaces of the frame members, and thus detract from the appearance of the finished frame.